For years scientists have wrestled with a puzzling fact: The universe appears to be remarkably suited for life. Its physical properties are finely tuned to permit our existence. Stars, planets and the kind of sticky chemistry that produces fish, ferns and folks wouldn't be possible if some of the cosmic constants were only slightly different.

Well, there's another property of the universe that's equally noteworthy: It's set up in a way that keeps everyone isolated.

We learned this relatively recently. The big discovery took place in 1838, when Friedrich Bessel beat out his telescope-wielding buddies to first measure the distance to a star other than the sun. 61 Cygni, a binary star in our own back yard, turned out to be about 11 light-years away. For those who, like Billy Joel, are fond of models, think of it this way: If you shrank the sun to a ping-pong ball and set it down in New York's Central Park, 61 Cygni would be a slightly smaller ball near Denver.

The distances between adjacent stars are measured in tens of trillions of miles. The distances between adjacent civilizations, even assuming that there are lots of them out there, are measured in thousands of trillions of miles – hundreds of light-years, to use a more tractable unit. Note that this number doesn't change much no matter how many planets you believe are studded with sentients – the separation distance is pretty much the same whether you think there are ten thousand galactic societies or a million.

Interstellar distances are big. Had the physics of the universe been different – if the gravitational constant were smaller – maybe suns would have been sprinkled far closer together, and a trip to your starry neighbors would have been no more than a boring rocket ride, kind of like cruising to Sydney. As it is, no matter what your level of technology, traveling between the stars is a tough assignment. To hop from one to the next at the speed of our snazziest chemical rockets takes close to 100,000 years. For any aliens who have managed to amass the enormous energy reserves and ponderous radiation shielding required for relativistic spaceflight, the travel time is still measured in years (if not for them, then for those they've left behind).

This has some obvious consequences (which, remarkably, have escaped the attention of most Hollywood writers.) To begin with, forget about galactic "empires" or more politically-correct "federations." Two thousand years ago, the Romans clubbed together an empire that stretched from Spain to Iraq, with a radius of about 1,200 miles. They could do this thanks to organization and civil engineering. All those roads (not to mention the Mediterranean) allowed them to move troops around at a few miles an hour. Even the most distant Roman realms could be reached in months or less, or about one percent the lifetime of your average legionnaire. It makes sense to undertake campaigns designed to hold together an extensive social fabric when doing so requires only a percent or so of a lifetime.

In the 19th century, steamships and railroads increased the troop travel speeds by a factor of ten, which extended the radius of control by a similar amount. The British could rule an empire that was world-wide.

But here's the kicker: Even if we could move people around at nearly the speed of light, this "one percent rule" would still limit our ability to effectively intervene – our radius of control – to distances of less than a light-year, considerably short of the span to even the nearest star other than Sol. Consequently, the Galactic Federation is a fiction (as if you didn't know). Despite being warned that Cardassian look-alikes were wreaking havoc and destruction in the galaxy's Perseus Arm, you couldn't react quickly enough to affect the outcome. And your conscripts would be worm feed long before they arrived on the front lines anyway.

In other words, aliens won't be getting in one another's face.

There's a similar argument to be made for communication. We seldom initiate information interchange that takes longer than months (an overseas letter, for instance). More generally, we seldom begin any well-defined project that lasts more than two or three generations. The builders of medieval cathedrals were willing to spend that kind of time to complete their gothic edifices, and those who bury time capsules are occasionally willing to let a hundred years pass before the canisters are dug up. But what about a project that takes several centuries, and possibly millennia? Who's willing to do that? Only Stewart Brand's "Long Now Foundation" seems to have the guts for this type of enterprise, proposing to build a clock that will keep time for ten thousand years.

Clearly, these simple observations must have implications for SETI which, as we noted, involves transmissions that will be underway for hundreds to thousands of years. In particular, if there are signals being bandied about the galaxy for purposes of getting in touch, either (1) the aliens are individually much longer-lived than we are, which – if you're a fan of circuit-board sentience – implies that they're probably not biological. Or (2) we're missing some important physics permitting faster-than-light communication, and extraterrestrial signaling efforts don't include burping light and radio waves into space.

Many readers will, in a display of endearing perversity, choose (2). Maybe they're right, but that flies in the face of what we know. And what we know argues something worth bantering about at your next cocktail party – namely, that the time scales for travel and communication are too long for easy interaction with beings whose lifetimes are, like us, only a century or less. So while the cosmos could easily be rife with intelligent life – the architecture of the universe, and not some Starfleet Prime Directive, has ensured precious little interference of one culture with another.